1. Field of the Invention
The present invention relates to an image display apparatus using a liquid crystal display element (particularly, reflective liquid crystal display element) such as a liquid crystal panel.
2. Description of the Related Art
An image projection device performs color separation and color composition by a polarizing beam splitter. The image projection device discussed in U.S. Pat. No. 6,626,540 divides unpolarized light emitted from a light source into a plurality of beams by a lens array to form a plurality of secondary light source images with each beam. The beams from the secondary light source image are superimposed on a liquid crystal panel by a condenser lens to illuminate the liquid crystal panel with nearly uniform brightness.
Each divided beam emitted from the lens array is incident on a plurality of polarization conversion cells corresponding to each lens cell of the lens array provided in a polarization conversion element. Each polarization conversion cell includes a polarization separation film, a ½ wavelength plate and a reflection plane. The unpolarized light incident on each polarization conversion cell is separated into P polarized light and S polarized light by the polarization separation film. The P polarized light passes through the polarization separation film. The ½ wavelength plate rotates polarization of P polarized light at 90 degrees. The P polarized light is emitted as the S polarized light.
On the other hand, S polarized light is reflected by the polarization separation film. The S polarized light reflects off a reflection plane and is emitted while it remains S polarized. The S polarized light whose direction of polarization is made uniform, is emitted from the polarization conversion element and is incident on the condenser lens.
Generally, in the polarization conversion element, it is difficult to convert the unpolarized light emitted from a light source having a limited size into linear polarized light with efficiency of 100%. For this reason, a polarizing plate is placed after the polarization conversion element to remove unnecessary polarized light. While such a polarizing plate has a function of removing the unnecessary light, the polarizing plate shields light having a normal direction of polarization (light having a desired direction of polarization) by 10% or more. Therefore, a projected image becomes slightly dark.
The polarization conversion element uniformly converts the direction of polarization of light to S polarized light. The S polarized light is separated into light in a red and a blue wavelength band, and light in a green wavelength band by a dichroic element. At this stage, the light in the red and the blue wavelength bands follows the same light path and has the same direction of polarization. Then, the light is passed through a wavelength (color) selective phase difference plate to lead each of the light in the red and the blue wavelength bands, to a red and a blue liquid crystal panel respectively using a polarizing beam splitter.
The wavelength selective phase difference plate converts the passing light in the red and the blue wavelength bands into light having the different direction of polarization respectively. Thus, an image projection device using the polarizing beam splitter generally employs a polarization conversion element placed close to a lens array, a polarizing plate, a dichroic element, a wavelength selective phase difference plate, and a polarizing beam splitter.
In the above-described configuration, in order to enhance the degree of polarization of illumination light incident on a liquid crystal panel, the polarizing plate needs to be arranged. However, when the polarizing plate is placed, an amount of the illumination light is reduced. On the contrary, in order to prevent reduction of the amount of the illumination light, it is considered to remove the polarizing plate. However, if the polarizing plate is removed, for example, light having the direction of polarization opposite to a desired direction is incident on a green liquid crystal panel, and light in a green wavelength band is incident on a red and a blue liquid crystal panel. In that case, green light having high spectral luminous efficiency is projected onto a screen. That projection of the green light reduces the contrast of an image. Further, the red light and the blue light may be incident on other liquid crystal panel than the liquid crystal panel corresponding to each of the red light and the blue light. Furthermore, the light having the direction of polarization different from the desired direction may be incident on the liquid crystal panel, which causes a reduction in contrast.